The invention concerns a method for the detection of stenosis in the blood circuit for extracorporeal blood treatment. The stenosis can be in the systemic blood circuit of the patient as well as in the extracorporeal blood circuit. The method evaluates pressure pulses produced either by the heart or by the blood pump of the extracorporeal circuit.
In one embodiment of the invention the pressure pulses in the extracorporeal circuit are measured non-invasively and without use of special pressure transducer membranes but directly on the tubing. The invention is explained more closely in the following description and in the patent claims.
Extracorporeal blood treatment today is a standard method, used especially for the treatment of end stage renal disease in the form of hemodialysis, hemodiafiltration or hemofiltration, but also employed for the treatment of hypercholesterinemia and for auto-immune diseases. Other extracorporeal treatment methods are based of physical effects, e.g., irradiation of blood with light, UV light or high-energy radiation (e.g., x-ray, beta- or gamma radiation). Furthermore, the application of increased or reduced temperature, and of magnetic, electric or electromagnetic fields is known.
For the extracorporeal treatment, a sufficiently effective access to the blood circuit of the body is required. For chronic treatment this access must remain patent for many years. The improvement of the treatment has resulted in many long-term patients. In addition the treatment of elderly patients became possible. Both are the causes that blood access becomes a growing problem and that it is often called the xe2x80x9cachilles heelxe2x80x9d of hemodialysis. The standard for blood access is a subcutaneous shunt between an artery and a vein. This shunt is created either by the connection of two blood vessels (Cimino-fistula) or with the help of an artificial vessel (graft). Blood accesses of this kind and the surgical techniques for their creation are described in all standard books on hemodialysis.
Some common problems with these blood accesses are stenosis, which are strictures of the vessels that may result in the total closure of the vessel and the loss of the fistula or graft if they are not detected and corrected timely. The word fistula will be used in the following for simplicity but the description is valid for grafts too.
The blood flow in a functional fistula is typically 700 ml/min with a wide range of 300-1500 ml/min. Larger blood flows have been measured too but there is a tendency to correct such blood accesses because high blood flow causes strain of the heart. In the extracorporeal circuit blood is commonly pumped with 200-500 ml/min. A blood flow in the fistula less than the blood flow in the extracorporeal circuit results in recirculation and therefore in a reduction of the efficacy of the method. It has been found that stenosis resulting in a blood flow of less than 600 ml/min in grafts will be followed by a total closure within short time. Early detection of this situation allows correction of stenosis before the total closure results in a total loss of the access. Because 600 mL/min are more than the commonly used flow in the extracorporeal circuit, it is not possible to detect this by a simple recirculation measurement. The fistula flow is therefore measured from time to time in order to detect stenosis timely. This is possible, e.g., with an ultraosound-doppler method requiring an expensive device and specialists. Such devices and knowledge are normally not available in dialysis units. An alternative method has been developed that measures recirculation with reversed blood flow in the extracorporeal circuit. Recirculation is then measured with a common method and the fistula flow is calculated. This method too requires a special device and trained personnel. Although it can be done in the dialysis unit it has not been adopted widely because it always requires an interruption of the treatment and is not cost effective therefore. This method is described in the U.S. Pat. No. 5,685,989.
Furthermore it has been recognized that the pressure measured in the extracorporeal circuit is influenced by stenosis in the fistula. Methods have been developed allowing detection of stenosis under certain circumstances. One method takes advantage of the empirical finding that information about stenosis downstream of the fistula can be gained from the venous pressure measured in the extracorporeal circuit provided a specific blood flow and specific cannulas are used. Stenosis is assumed when the venous pressure is above a limit evaluated by the clinic before. A description of the method is published in, e.g.,: Schwab S J Raymond J R Saeed M Newman G E Dennis P A Bollinger R R. Prevention of hemodialysis fistula thrombosis. Early detection of venous stenoses. Kidney International 1989;36:707-11.
This method has obvious disadvantages: First the empirical evaluation of the limit requiring either the acceptance of thrombosis during the evaluation period or, alternatively requiring comparison with a reference method. Further the use of a specific cannula and a specific blood flow for the measurement. This means that the treatment is influenced at least temporarily.
Another method measures the static fistula pressure. This is done with special pressure sensors positioned at the level of the fistula that are connected to the fistula either through a special cannula or through the extracorporeal circuit. Also, it is known to use the sensors already employed by the extracorporeal circuit. In this case it is necessary to correct for the height difference between the liquid level in the extracorporeal circuit and the fistula. This method too requires personnel time for the measurement of the height difference and the evaluation of the measurement. Because the patient can alter his position such a measurement can be done only once if personnel time is limited.
This method is described in: Besarab A, Al-Saghir F, Alnabhan N, Lubkowski T. Frinak S. Simplified Measurement of Intra-Access Pressure. ASAIO Journal 1996;42:M682-7
Stenosis can develop in the extracorporeal circuit too. These are flow resistances in cannulas that have not been taken into account correctly by the user or, alternatively, flow resistances caused by kinks in the tube, pinched tubing or by blood clotting. This stenosis may cause a reduction of the effective blood flow in the extracorporeal circuit and therefore a reduction of the efficacy or to hemolysis because of the high shear force at the point of the stenosis. To detect such stenosis pressure sensors in the extracorporeal circuit are employed that work invasively, this means they are connected through tubing or, alternatively, sensors working with special pressure transmitters. These sensors are expensive and promote blood clotting. They measure mean pressure which means that the pressure pulses in the extracorporeal circuit are smoothed by electronic hardware or software and therefore eliminated. Another patent of the inventor (DE 3806248) describes a method employing the pressure pulses for monitoring the proper function of the pressure monitor. DE 3806248 also mentions the possibility to measure the pulse frequency of the patient with this method. DE 3806248 furthermore describes a method to detect stenosis between the blood pump and the venous pressure sensor from a phase shift.
The purpose of the new invention is to describe a simple method and a device for the early detection of stenosis in the vicinity of the fistula and for continuous monitoring of the extracorporeal circuit. Additionally it is possible to measure the rotational speed of the blood pump and the pulse frequency of the patient already described previously. The method is based on the amplitude of the pressure pulses, corrected if necessary by a frequency dependent function, measured in the extracorporeal circuit with the blood pump in operation or stopped. Measurements in the clinic have shown that, with the blood pump stopped, it is possible to measure the patient""s pulse downstream of the blood access in the extracorporeal circuit. It is not possible to do this with the pressure monitors of dialysis machines because these are artificially dampened as already described. Rather the observations have been made with not dampened pressure sensors. Pressure sensors of common dialysis machines are able to measure such pressure pulses too provided the electronic damping is removed.
It has been recognized surprisingly that the pulse amplitudes are higher with stenosis. Also it has been recognized that the pulses from the patient""s pulse can still be recognized even with the blood pump running in case of stenosis. When the extracorporeal circuit was in operation it was not possible to find a correlation between the pressures measured before the blood pump (p(art)) and in the venous drip chamber (p(ven)) and the pressure pulses produced by the blood pump. Surprisingly it was found that an approximately linear correlation exists between the amplitude of the pressure pulses and the mean pressure if the compliance of the system is reduced by removal of the air from the system.
As already mentioned pressure monitoring in the extracorporeal system is done with systems employing partially filled pressure lines or with systems using special pressure transmitter membranes. Attempts have been made to measure the pressure directly on the tubing, this means that the tubing wall is used for pressure transmittance. This has failed with the commonly used PVC tubing because of creeping. DE 4106444 describes a method reducing the creeping effect by a mechanical bias of the tubing during storage. Surprisingly it has been found that the pressure pulses can be measured with an xe2x80x9cacoustic contact sensorxe2x80x9d of the company Apollo Research Corp. Denew, N.Y., USA in the frequency range of interest. This sensor has the shape of a flat cylinder that is positioned directly on the tubing which is only slightly compressed.
To limit the pressure produced by the blood pump in the extracorporeal circuit peristaltic pumps are normally so designed that the occlusion of the rollers is reduced above a typical pressure of 1-2 bar which limits the pressure. At this occasion, the fluid flows backward temporarily through the only partially occluded tube into the sucking area. If blood is pumped, it will be hemolysed by the shear forces which may result in life threatening complications. Surprisingly it has been found that this situation can be detected by an increase of the pulse amplitude as well downstream as upstream of the blood pump although the mean pressure does not increase downstream because of the flow limitation while upstream (on the sucking side) a reduction of the mean sucking pressure is measured.